Don't set locale in headers test.

tests/test_headers.cpp

@@ -29,8 +29,6 @@ X-UmläÜt: 🙂
     bool exception = false;
     string value;
 
-    std::locale::global(std::locale("de_DE.UTF-8"));
-
     WHEN("We search for “cache-control”")
     {
         try
@@ -53,6 +51,8 @@ X-UmläÜt: 🙂
 
     // WHEN("We search for “X-UMLÄÜT”")
     // {
+    //     std::locale::global(std::locale("de_DE.UTF-8"));
+    //
     //     try
     //     {
     //         value = ret.get_header("X-UMLÄÜT");